Geothermal heat harvesters

ABSTRACT

Thermal energy is extracted from geological formations using a heat harvester. In some embodiments, the heat harvester is a once-through, closed loop, underground heat harvester created by directionally drilling through hot rock. The extracted thermal energy can be converted or transformed to other forms of energy.

CROSS-REFERENCE

This application is a continuation of PCT Application No.PCT/US2016/053569, filed Sep. 23, 2016, which claims the benefit ofprovisional U.S. Application No. 62/232,271, filed on Sep. 24, 2015,each of which is entirely incorporated herein by reference.

BACKGROUND

Conventional hydrothermal geothermal energy may be extracted from theearth by drilling a well into an underground aquifer which has beenheated by a co-located magmatic intrusion. The heated water may beextracted from the aquifer in the form of steam, heated brine, or amixture of both, and is typically used to generate electricity. Whilehigh temperature rock (e.g., above 200° C.) exists pervasively in theearth's crust within commercially accessible depths (e.g., within 10km), such resources may be only rarely co-located with naturalunderground reservoirs of water. Hydrothermal geothermal resources mayalso be subject to a depleting water inventory unless such inventory isreplenished by reinjecting fluids, which poses difficulties in regionswhere the water supply is limited.

The rarity of a naturally co-located heat and fluid resource has led tothe development of engineered, or enhanced, geothermal systems (EGS).EGS may create a network of cracks within a body of hot rock throughhydraulic fracturing, and then introduce water into the newly createdcracks to extract the heat from the rock. The advantage of EGS is thatwater reservoirs may be artificially created within a geological heatresource. However, EGS suffers from difficulties in controlling thefracture network pathway, from a loss of injected fluid, from leachingof minerals from the rock, and from declining heat extraction due to therapid local cooling of rock around the fractures.

SUMMARY

Recognized herein is a need for improved systems and methods forextracting heat from geological formations (also “geothermal heatharvesting” herein). The present disclosure provides closed loop systemsfor generating geothermal power by extracting heat from a body of rock,such as by drilling a borehole and encasing it with a pipe system andsubsequently directing a working fluid (e.g., water) through the pipesystem. The surrounding rock transfers heat to at least a portion of thepipe system. Cold working fluid is directed (e.g., pumped) into the pipesystem and heated up by the surrounding rock as it flows through a heatharvesting portion of the pipe system. Such systems can advantageouslykeep the working fluid contained within a closed loop, with reduced orminimal risk of loss of the working fluid to the surrounding rock andsubstantially none of the environmental issues associated with waterextraction from rock formations.

In some embodiments, a once-through, closed loop, underground heatharvester (also “heat harvester” herein) is created by directionallydrilling at least a portion of the heat harvester through hot rock. Twoor more independently drilled portions of the heat harvester (e.g.,comprising two or more independently drilled well portions) can beconnected with one or more couplings to create a complete loop of theunderground heat harvester. Conductive cement or grout can be used toincrease or improve flow of heat into the heat harvester. For example,conductive cement or grout can be used to increase or improve flow ofheat into the heat harvester in a primary heat transfer region.Insulating cement or grout can be used to decrease or prevent loss ofheat from the heat harvester whenever the fluid is hotter than thesurrounding rock. Directional drilling can allow heat extraction to beincreased or maximized by creating a heat harvester with sufficientlength (and/or width) within a high temperature portion of the targetformation. A field of heat harvesters can be shaped in various ways toincrease or maximize heat extraction as a system. Heat harvesters may bedirectionally drilled through rock with known high conductivity toincrease or maximize thermal production. Heat harvesters of thedisclosure can be used, for example, to sustainably extract thermalenergy (also “heat” herein) from geological formations. The extractedthermal energy can be converted or transformed to other forms of energy.

An aspect of the disclosure relates to a geothermal heat harvestingsystem, comprising a closed fluid flow path having a first segment, asecond segment and a third segment, wherein the first segment, secondsegment and third segment are disposed below a surface such that, duringuse, a working fluid is directed through the closed fluid flow pathalong a direction that includes, in sequence, the first segment, secondsegment and third segment, wherein the closed fluid flow path includes afluid entrance and a fluid exit that may be co-located at the surface,and wherein (i) the second segment may be substantially free of thermalinsulation, (ii) the third segment may be thermally insulated, and (iii)the first segment and third segment may be oriented at an angle greaterthan about 0° with respect to the second segment.

In some embodiments, the second segment of the geothermal heatharvesting system comprises a conductive cement or grout. In someembodiments, the third segment of the geothermal heat harvesting systemcomprises an insulating cement or grout. In some embodiments, thegeothermal heat harvesting system comprises one or more deviated fluidflow paths.

In some embodiments, the geothermal heat harvesting system comprises oneor more turbines for generating power with the aid of thermal energyform the working fluid. In some embodiments, the geothermal heatharvesting system comprises one or more heat exchangers for extractingthermal energy from the working fluid. In some embodiments, the workingfluid remains in a liquid phase in the one or more heat exchangers. Insome embodiments, the one or more heat exchangers are located at thesurface. In some embodiments, the thermal energy extracted from theworking fluid is used for co-generation.

In some embodiments, the geothermal heat harvesting system comprisespower plant equipment at the surface for industrial use of thermalenergy from the working fluid. In some embodiments, the industrial useincludes power generation. In some embodiments, the industrial useincludes district heating.

In some embodiments, the geothermal heat harvesting system comprises aclosed fluid flow path drilled through virgin rock. In some embodiments,at least a portion of the closed fluid flow path is directionallydrilled. In some embodiments, at least a portion of rock surrounding theclosed fluid flow path is selectively targeted.

In some embodiments, the working fluid of the geothermal heat harvestingsystem is directed through a closed fluid flow path without undergoing aphase change. In some embodiments, the working fluid undergoes a phasechange. In some embodiments, the working fluid comprises a pressurizedliquid.

In some embodiments, the geothermal heat harvesting system comprises afirst segment that is substantially free of thermal insulation. In someembodiments, the system further comprises a fourth segment between thefirst segment and the second segment or between the second segment andthe third segment.

In some embodiments, the geothermal heat harvester comprises a secondsegment disposed at a depth of at least about 0.5 kilometers withrespect to the surface. In some embodiments this depth is substantiallyconstant across a length of the second segment.

In some embodiments, the geothermal heat harvesting system comprises aclosed fluid flow path that comprises a heat exchange region below thesurface. In some embodiments, the closed fluid flow path comprises aheat exchange region below the surface with a horizontal length of aleast about 500 meters. In some embodiments, the second segment of thegeothermal heat harvesting system has a horizontal length of at leastabout 500 meters. In some embodiments, the working fluid of thegeothermal heat harvesting system is directed once through the closedfluid flow path.

An aspect of the disclosure is directed to a geothermal heat harvestingsystem, comprising a geothermal heat harvester comprising an entranceand an exit at a surface, wherein the entrance and the exit are in fluidcommunication via a path. The path may comprise a first segmentextending between the surface and a first depth, the first segmentcomprising the entrance; a second segment in fluid communication withthe first segment and extending between the first depth and a seconddepth, the second segment being at an angle with respect to the firstvertical segment of at least about 5°; a third segment at the seconddepth that is in fluid communication with the second segment, whereinthe third segment comprises a heat transfer region; a fourth segment influid communication with the third segment and extending between thesecond depth and the first depth; and a fifth segment in fluidcommunication with the fourth segment and extending between the firstdepth and the surface, the fifth segment comprising the exit, whereinthe fourth segment is at an angle with respect to the fifth segment ofat least about 5°.

In some embodiments, the first segment and the fifth segment of thegeothermal heat harvesting system are each substantially vertical, andthe third segment is substantially horizontal.

In some embodiments, the path of the geothermal heat harvesting systemcomprises two independently drilled portions connected with a coupling.In some embodiments, a first of the two independently drilled portionscomprises the first segment, the second segment and a first portion ofthe third segment, and a second of the two independently drilledportions comprises a second portion of the third segment, the fourthsegment and the fifth segment. In some embodiments, the path comprisestwo or more independently drilled portions connected with one or morecouplings.

In some embodiments, the geothermal heat harvesting system furthercomprises an additional geothermal heat harvester arranged in a radiatorconfiguration with the geothermal heat harvester, the additionalgeothermal heat harvester comprising: a sixth segment in fluidcommunication with at least a portion of the second segment, the sixthsegment extending between a deviation depth and the second depth anddeviating from the second segment at a first angle; a seventh segment atthe second depth that is in fluid communication with the sixth segment,the seventh segment being substantially horizontal, wherein the seventhsegment and the third segment are spaced apart and substantiallyparallel; and an eighth segment in fluid communication with the seventhsegment and at least a portion of the fourth segment, the eighth segmentextending between the second depth and the deviation depth and deviatingfrom the fourth segment at a second angle. In some embodiments, thefirst angle is substantially the same as the second angle.

In some embodiments, the path of the geothermal heat harvesting systemforms a closed loop. In some embodiments, the path comprises twoportions, wherein a first of the two portions comprises a well head thatforms the entrance, and wherein the second of the two portions comprisesa well head that forms the exit. In some embodiments, the geothermalheat harvesting system further comprises insulating cement or groutalong at least a portion of the path, conductive cement or grout alongat least a portion of the path, or a combination thereof. In someembodiments, the entrance and exit of the geothermal heat harvestingsystem are co-located. In some embodiments, the geothermal heatharvesting system further comprises a primary fluid that flows throughat least a portion of the path, wherein the primary fluid enters at theentrance and exits at the exit, and wherein a flow rate of the primaryfluid is controlled over life of the geothermal heat harvester such thata heat extraction rate from the geothermal heat harvester is leveledthrough life.

In some embodiments the geothermal heat harvesting system furthercomprises an additional geothermal heat harvester operating togetherwith the geothermal heat harvester in a field configuration, theadditional geothermal heat harvester having a separate path withsubstantially the same configuration as the path of the geothermal heatharvester. In some embodiments, the additional geothermal heat harvesteris adjacent to the geothermal heat harvester, wherein the geothermalheat harvester circulates a first primary fluid and the additionalgeothermal heat harvester circulates a second primary fluid, and whereinthe first primary fluid and the second primary fluid circulate in acounter-flow configuration with respect to each other. In someembodiments, the additional geothermal heat harvester and the geothermalheat harvester are spaced to prevent cooling overlap.

Another aspect of the disclosure is directed to a geothermal heatharvesting system, comprising a geothermal heat harvester comprising anentrance and an exit at a surface, wherein the entrance and the exit arein fluid communication via a path. The path may comprise a first segmentextending between the surface and a target depth, the first segmentcomprising the entrance; a second segment in fluid communication withthe first segment and extending radially outward at the target depth,the second segment being substantially perpendicular to the firstsegment; a third segment in fluid communication with the second segmentand extending in an arc at the target depth; a fourth segment in fluidcommunication with the third segment and extending radially inward atthe target depth; and a fifth segment extending between the target depthand the surface, the fifth segment comprising the exit and beingsubstantially perpendicular to the fourth segment. The second segment,the third segment and the fourth segment together comprise a primaryheat transfer region.

In some embodiments, the first segment and the fifth segment of thegeothermal heat harvesting system are each substantially vertical, andthe second segment and the fourth segment are each substantiallyhorizontal.

In some embodiments, the path of the geothermal heat harvesting systemcomprises two independently drilled portions connected with a coupling.In some embodiments, the coupling is at the target depth. In someembodiments, the path comprises two or more independently drilledportions connected with one or more couplings. In some embodiments, theone or more couplings are at the target depth.

In some embodiments, the geothermal heat harvesting system furthercomprises: a sixth segment between the first segment and the secondsegment, the sixth segment deviating from the first segment toward thesecond segment; and a seventh segment between the fourth segment and thefifth segment, the seventh segment deviating from the fifth segmenttoward the fourth segment. In some embodiments, the geothermal heatharvesting system further comprises an additional geothermal heatharvester deviating from the geothermal heat harvester, the additionalgeothermal heat harvester comprising: an eighth segment in fluidcommunication with the first segment; a ninth segment in fluidcommunication with the eighth segment and extending radially outward atthe target depth, the ninth segment being substantially perpendicular tothe first segment and the eighth segment deviating from the firstsegment toward the ninth segment; a tenth segment in fluid communicationwith the ninth segment and extending in an arc at the target depth; aneleventh segment in fluid communication with the tenth segment andextending radially inward at the target depth, the eleventh segmentbeing substantially perpendicular to the fifth segment; and a twelfthsegment in fluid communication with the eleventh segment and the fifthsegment, the twelfth segment deviating from the fifth segment towardeleventh segment. In some embodiments, the ninth segment and theeleventh segment are each substantially horizontal. In some embodiments,the horizontal portions of the geothermal heat harvester and theadditional geothermal heat harvester are rotated by about 45°, 90° or135° from each other.

In some embodiments, the path of the geothermal heat harvesting systemforms a closed loop. In some embodiments, the path comprises twoportions, wherein a first of the two portions comprises a well head thatforms the entrance, and wherein the second of the two portions comprisesa well head that forms the exit. In some embodiments, the geothermalheat harvesting system further comprises insulating cement or groutalong at least a portion of the path, conductive cement or grout alongat least a portion of the path, or a combination thereof. In someembodiments, the entrance and exit of the geothermal heat harvestingsystem are co-located. In some embodiments, the geothermal heatharvesting system further comprises a primary fluid that flows throughat least a portion of the path, wherein the primary fluid enters at theentrance and exits at the exit, and wherein a flow rate of the primaryfluid is controlled over life of the geothermal heat harvester such thata heat extraction rate from the geothermal heat harvester is leveledthrough life.

In some embodiments the geothermal heat harvesting system furthercomprises an additional geothermal heat harvester operating togetherwith the geothermal heat harvester in a field configuration, theadditional geothermal heat harvester having a separate path withsubstantially the same configuration as the path of the geothermal heatharvester. In some embodiments, the additional geothermal heat harvesteris adjacent to the geothermal heat harvester, wherein the geothermalheat harvester circulates a first primary fluid and the additionalgeothermal heat harvester circulates a second primary fluid, and whereinthe first primary fluid and the second primary fluid circulate in acounter-flow configuration with respect to each other. In someembodiments, the additional geothermal heat harvester and the geothermalheat harvester are spaced to prevent cooling overlap.

Additional aspects and advantages of the present disclosure will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only illustrative embodiments of thepresent disclosure are shown and described. As will be realized, thepresent disclosure is capable of other and different embodiments, andits several details are capable of modifications in various obviousrespects, all without departing from the disclosure. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings or figures (also “FIG.” and “FIGs.” herein), ofwhich:

FIG. 1 is a schematic of example surface operations;

FIG. 2 is an elevation view of a triangle configuration;

FIG. 3 is an isometric view of a triangle configuration with multipleindividual heat harvesters in a field configuration;

FIG. 4 is an isometric view of a triangle configuration with portions ofindividual heat harvesters deviating from common entrance and exitportions;

FIG. 5 is an isometric view of a flower petal configuration withmultiple individual heat harvesters in a field configuration;

FIG. 6 is an isometric view of a flower petal configuration withportions of individual heat harvesters deviating from common entranceand exit portions; and

FIG. 7 is an elevation view of a closed loop heat harvesterdirectionally drilled through a vein of rock with high thermalconductivity.

DETAILED DESCRIPTION

Described herein are systems and methods for extracting heat fromgeological formations using a geothermal heat harvester (also “heatharvester” herein). The heat harvester can comprise a casing. The heatharvester can contain a fluid (e.g., a working fluid). The casing cancomprise a pipe (or pipe system) through which the heat harvesting fluidflows. The casing can form a closed fluid flow path (also “closed loop”herein). The casing can be installed inside a hole (or a system ofholes) created through drilling (also “borehole” herein). The heatharvester can comprise one or more portions. For example, the heatharvester can comprise at least 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10portions. For example, the heat harvester can comprise a heat harvestingportion connected to a well portion via one or more connecting portions.Each such portion can comprise a portion of the casing. The portions maybe substantially linear or substantially non-linear (e.g., arced, angledor curved). At least a subset of the portions may be formed at least inpart using directional drilling.

The well portion can comprise an entrance well portion, an exit wellportion or both. For example, a well portion can comprise the entrancewell portion and the exit well portion. The entrance and exit wellportions can each comprise a well head. A well portion may be configuredfor a single heat harvester or may be shared by multiple heatharvesters.

Subsets of the portions may be grouped into larger portions. An entranceportion may comprise a well portion (e.g., an entrance well portion) andone or more connecting portions. An exit portion may comprise one ormore connecting portions and a well portion (e.g., an exit wellportion). In some cases, the entrance portion and/or the exit portionmay (e.g., each) further comprise a portion of the heat harvestingportion. At least a subset of the portions may be joined or connectedwith a coupling (e.g., a coupling between the respective casings). Forexample, the entrance portion and the exit portion may be connected witha coupling (e.g., a coupling between two portions of the heat harvestingportion, or a coupling between a connecting portion and the heatharvesting portion).

The well portion can be the initial portion of the undergroundinstallation. The remainder of the heat harvester may be drilled fromthe well portion. The well portion may comprise multi-stage casingcemented or grouted in a conventional way. From the bottom of the wellportion, the casing can enter into the borehole in the rock. The wellportion can comprise one or more such casings. In the borehole in therock, the casing may be surrounded by an insulating cement or grout downto a rock temperature depth referred to as “heating depth”. Below theheating depth, a heat harvesting portion of the casing can collect orharvest the heat from the surrounding rock. The heat harvesting portionof the casing may be enclosed by a conductive cement or grout. At leasta portion of the heat harvesting portion can extend through a primaryheat transfer region. Past the primary heat transfer region, the casingmay extend back up to the well portion. Above a rock temperature depthreferred to as “insulation depth”, the casing may be enclosed by aninsulating cement or grout and may not be harvesting additional heat.

The heat harvester can comprise a closed loop heat harvester. Thesurrounding rock can transfer heat to the heat harvester walls (e.g.,the casing in a heat harvesting portion) via, for example, conduction. Aonce-through design, where liquid enters at one well head, flows througha length of pipe and exits from a separate well head, may beadvantageous over other methods or configurations. The heat harvestermay be a once-through, closed loop, directionally drilled heatharvester. The once-through, closed loop, underground heat harvester canbe directionally drilled through a geological formation (e.g., rock).The geological formation can be a high temperature rock. The heatharvester may be drilled through virgin rock.

The heat harvester can have an entrance (e.g., an inlet to the entrancewell portion) and an exit (e.g., an outlet of the exit well portion).The entrance and/or the exit can be located at the rock surface. Theentrance and exit of the underground heat harvester can be positionednear each other (e.g., as co-located well heads) to allow, for example,co-located operation of the system and/or a centralized drillingoperation during heat harvester creation. The entrance and exit of theheat harvester may be spaced apart to allow, for example, locating theentrance near a fluid source while locating the exit near a desired heatload. A combination of co-located and spaced apart configurations may beused (e.g., when a given entrance has multiple exits).

The heat harvester can have an entrance portion and an exit portion. Theentrance portion can comprise the entrance well portion. The entranceportion can further comprise one or more connecting portions. The exitportion can comprise the exit well portion. The exit portion can furthercomprise one or more exit well portions.

Extraction of heat from the earth's crust can be accomplished by theflow (e.g., pumped and/or through natural circulation) of a workingfluid (e.g., a liquid working fluid, such as, for example, pressurizedwater or a long-chain hydrocarbon) into a closed loop heat harvesterinstalled in rock of elevated temperature. The fluid can be injected(e.g., at the rock surface) at a low temperature, be gradually heated asit travels through the heat harvester, and exit at a temperature closeto that of the rock. The heat harvester can be a closed loop to preventfluid loss. In an example configuration where the heat harvester isfully cased, the environmental impact stemming from leaching the hostrock may be decreased or eliminated.

The working fluid can be heated as it travels through the heat harvester(e.g., in a loop). At least a portion of the working fluid (also“process fluid,” “fluid,” “primary fluid” and “heat harvesting fluid”herein) may be pressurized. For example, heated primary fluid can beunder pressure sufficient to prevent boiling within the closed loop pipesystem. Once at the surface, the heated primary fluid within the loopmay be kept liquid by exchanging the harvested heat with a secondaryprocess fluid. Alternatively or in combination, the heated primary fluidmay be flashed to vapor (e.g., steam) at the surface. Surface equipment(e.g., surface power plant equipment or other surface mounted equipment)may be used to extract and/or convert energy from the heated primaryfluid. The energy from the heated primary fluid may have various uses,including, but not limited to, electrical power generation,desalination, use as a high temperature heat source for industrialprocesses, co-generation, district heating and/or cooling, or anycombination thereof. The energy harvested from the heated primary fluidmay be from about 5 megawatts to 1 gigawatt. For example, the heatedprimary fluid may be directed to a turbine to generate power, or used toheat a secondary fluid which may be directed to a turbine to generatepower.

The heat harvester can comprise an open loop heat harvester. In an openloop heat harvester configuration, the working fluid between an entranceand an exit of the heat harvester may flow in a closed piping system,and upon flowing through the exit of the heat harvester may be used bysurface equipment to extract and/or convert energy from the workingfluid. In some instances, the process of extracting and/or convertingenergy from the working fluid may lead to the working fluid not beingfully recoverable; in such instances a source of new working fluid maybe provided to supplement the working fluid lost due to the extractingand/or converting.

FIG. 1 is a schematic of example surface operations, showing two examplemethods of transferring heat extracted from an underground heatharvester 100. Surface operations may use surface mounted equipment(e.g., no underground pumps, no underground turbines and/or nounderground valves). The surface mounted equipment may comprise surfacemounted power plant equipment. The underground heat harvester may extendto a given depth. The depth may be defined as the distance measured fromthe surface of the rock in a vertical direction (i.e., a directionparallel to the gravity vector). A heated primary fluid 101 may bemaintained in a liquid phase underground (e.g., no boiling while theprimary fluid is underground in the heat harvester 100). In a firstmethod (top), the heated primary fluid 101 may remain in a liquid state(e.g., does not undergo phase change from liquid to gas) upon exitingthe underground heat harvester 100 (e.g., the heated primary fluid 101remains in a liquid state in a heat exchanger). The heated primary fluid101 exiting the underground heat harvester 100 may enter a heatexchanger (e.g., a surface heat exchanger) 104 and heat a coolersecondary process fluid 102 into a heated secondary process fluid 103.The heated primary fluid may remain in a liquid phase in the heatexchanger. The heated primary fluid 101 cools off in the surface heatexchanger 104 and then may enter a pressurizing tank 105. Thepressurizing tank 105 may be a source for an injection pump 106, whichmay pump cooled primary fluid 107 back into the underground heatharvester 100. In a second method (bottom), heat may be transferred byflashing a heated primary fluid 108 exiting the underground heatharvester 100 through a throttling valve 109 into a flash drum 110,where steam is drawn off for plant use 111. A separate source of fluidor return fluid from the plant 112 may then be pressurized through aninjection pump 113 which pumps cooled primary fluid 114 into theunderground heat harvester 100.

Heat may be transferred to the heat harvester (e.g., to the heatharvesting portion of the heat harvester) via various heat transfermechanisms (e.g., conduction, convection and/or radiation). Heat fromdistant rock may be transmitted to the heat harvester throughconduction. Heat may be transmitted to the heat harvester throughnatural convection of water within a porous, water-saturated rock (e.g.,after significant temperature gradients are developed around the heatharvester). Heat transfer to the heat harvester through convection offluid(s) (e.g., water and/or other gaseous or liquid fluids) may beenhanced (e.g., by augmenting or cracking the surrounding rock prior tocasing the well). The conductive property of rock surrounding the heatharvester may be enhanced through injection of conductive material(e.g., cement or grout). The injection may be implemented whencompleting the heat harvester (e.g., when completing the well).

The performance of the heat harvester can be improved by configuring theheat harvester (e.g., the heat harvesting portion) to given (e.g.,local) geological and/or heat supply conditions (e.g., by shaping thedesign of the heat harvester to improve or optimize local geological andheat supply conditions). For example, diameter, borehole length and/orother characteristics of the heat harvester may be configured accordingto given geological and/or heat supply conditions. The heat harvester'sheat transfer surface area (e.g., the surface area of the heatharvesting portion) can be increased by increasing the diameter, byextending the borehole length, or both. Increasing the borehole lengthcan increase the volume of accessed rock. With recent improvements indrilling technology driven by the oil and gas industry resulting insignificant reductions in drilling cost, increased drilling rates andincreased precision in the control of directional drilling, limitationsof rock thermal conductivity may be offset (e.g., at least partiallyoffset) by creating a long, underground heat harvester.

The heat harvester (or individual portions thereof) can have a givendiameter (e.g., borehole diameter or casing diameter). The diameter ofthe heat harvester may be similar to borehole and/or casing sizes of atypical oil and gas or geothermal well. The diameter of various portionsof the heat harvester may or may not be the same. For example, aborehole in a well portion may be greater than or equal to a borehole ina heat harvesting portion (e.g., since a borehole in the well portionmay comprise one or more casings while a borehole in the heat harvestingportion may comprise a single casing). A diameter of the borehole can begreater than or equal to about 8 inches. In some cases, the diameter ofthe borehole may be significantly greater than 8 inches (e.g., as aresult of improved drilling technology). In some examples, the diameterof the borehole may be greater than or equal to about 6 inches, 7inches, 7⅝, 8 inches, 9 inches, 10 inches, 11 inches, 12 inches, or 36inches. A diameter of the casing may be less than or equal to about 7⅝inches. In some cases, the diameter of the casing may be significantlyless than 7⅝ inches (e.g., where coil-tubing utilization in the heatharvesting portion is feasible). In some examples, the diameter of thecasing may be less than or equal to about 8 inches, 7⅝ inches, 7 inches,6 inches, 5 inches, 4 inches or 3 inches (e.g., the diameter in the wellportion may vary in accordance with American Petroleum Institute (API)standards for oil well casings).

Heat harvesters can be installed in (e.g. drilled through) rock of atarget temperature. The target temperature may be relatively constant ata target depth. The target temperature and depth may define a primaryheat transfer region (also “target heat exchange region” herein). Thetarget temperature may be, for example, between about 100° C. and about500° C. The target temperature may also be, for example, between about200° C. and about 400° C. The target temperature may be at least about100° C., 150° C., 200° C., 250° C., 300° C., 350° C. or 400° C. Thetarget temperature may be less than about 500° C., 450° C., 400° C.,350° C., 300° C., 250° C. or 200° C. The target depth (e.g., for theprimary heat transfer region) may be, for example, between about 0.5kilometers (km) and about 12 km, between about 2 kilometers (km) andabout 7 km, or between about 2 km and 10 km. The target depth may be atleast about 0.5 km, 1 km, 2 km, 3 km, 4 km, 5 km, 6 km, 7 km, 8 km, 9km, 10 km or 12 km. The target depth may be less than about 12 km, 11km, 10 km, 9 km, 8 km, 7 km, 6 km, 5 km, 4 km, 3 km, 2 km, 1 km or 0.5km.

The primary heat transfer region can comprise a rock region at a givendepth (also “target depth” herein) and having a given horizontal length(e.g., horizontal length in a direction parallel to the plane of view inFIG. 2) and/or a given horizontal width (e.g., horizontal width in adirection perpendicular to the plane of view in FIG. 2). A geothermalheat harvester or a plurality of geothermal heat harvesters may targetat least a portion of the primary heat transfer region (e.g., along thehorizontal length, the horizontal width or both). The primary heattransfer region may or may not be centered with respect to the center ofthe heat harvester(s) and/or with respect to the location of the wellentrance and/or exit portion(s)).

The length (and/or width) of the underground heat harvester may be a keydriver of thermal performance. The length (and/or width) of theunderground heat harvester can be extended as needed through directionaldrilling. Directional drilling can allow heat extraction to be increasedor maximized by creating a heat harvester with sufficient length (and/orwidth) within a high temperature portion of the target formation (e.g.,the primary heat transfer region). A heat harvester (e.g., a heatharvesting portion of the heat harvester) may have a horizontal length(and/or width) in the primary heat transfer region that is as long aseconomically feasible (e.g., related to horizontal drilling costs atdepth). Such horizontal length (and/or width) in the primary heattransfer region may be configured based on geological considerations(e.g., avoidance of fault-lines, certain rock formations and/or propertyboundary lines). Thus, the horizontal length (and/or width) in theprimary heat transfer region may be arbitrarily long subject to, forexample, the aforementioned constraints. The horizontal length (and/orwidth) in the primary heat transfer region may vary depending on rockproperties and heat supply conditions. A heat harvester may have ahorizontal length (and/or width) in the primary heat transfer region ofgreater than or equal to about 100 meters (m), 200 m, 300 m, 500 m, 600m, 700 m, 800 m, 900 m, 1 kilometer (km), 1.2 km, 1.4 km, 1.6 km, 1.8km, 2 km, 2.5 km, 3 km, 3.5 km, 4 km, 4.5 km, 5 km, 5.5 km, 6 km, 6.5km, 7 km, 7.5 km, 8 km, 8.5 km, 9 km, 9.5 km or 10 km. The horizontallength (and/or width) in the primary heat transfer region may be betweenabout 3 km and 5 km, or between about 3 km and 10 km. The horizontallength (and/or width) in the primary heat transfer region may be greaterthan 10 km (e.g., if two separate directionally drilled portions of theheat harvester (e.g., comprising two separate directionally drilled wellportions) are connected). In some cases, the horizontal length (and/orwidth) in the primary heat transfer region may be significantly greaterthan 10 km (e.g., as a result of improved drilling and casinginstallation technology).

The layout of the heat harvester (also “underground heat harvester”herein) can be configured to increase or optimize performance within agiven (e.g., particular) geological resource. For example, the heatharvester may be drilled in a triangular shape or in a flower petalshape, as described in greater detail elsewhere herein.

Multiple heat harvesters may operate together. As described in greaterdetail elsewhere herein, multiple heat harvesters may operateindependently (e.g., each comprising separate entrance, exit and/orother portions), co-dependently (e.g., at least a portion of the heatharvesters can share entrance, exit and/or other portions), or acombination thereof. In some embodiments, multiple heat harvesters mayoperate independently in a field configuration. A field of heatharvesters can be shaped in various ways to increase or maximize heatextraction as a system (e.g., the heat harvesting portions can be shapedto increase or maximize heat extraction as a system). In someembodiments, multiple heat harvesters may operate co-dependently in adeviated configuration.

Heat harvesters may be constructed individually and/or in groups. Heatharvesters may be drilled in groups for economies of scale (e.g., in afield configuration). When grouped, heat harvesters can be drilled apartfrom each other at a suitable distance (e.g., at a distance sufficientto prevent overlapping regions of heat harvesting). For example, whengrouped, deviating portions of individual heat harvesters can besuitably spaced apart in a horizontal direction (e.g., to preventcooling overlap). Where portions of the heat harvesters are operatingadjacent to each other, they may be operated in a counter-flowconfiguration. Operation in the counter-flow configuration may beadvantageously used to offset thermal gradient(s) developed along theheat harvesters. For example, where heat harvesters (e.g., entrance andexit portions of the heat harvesters) are operating adjacent to eachother, they may be operated in a counter-flow configuration (e.g., tooffset axial thermal gradients developed along the length of each heatharvester due to the rock cooling more at respective entrances than atrespective exits). In this example, operation of two adjacent portionsof the heat harvesters (e.g., operation of two sets of adjacent entranceand exit portions) in the counter-flow configuration may createcomplementing axial thermal gradients along their lengths. Due to theworking fluid (e.g., water) being colder at the injection point(entrance) of an individual heat harvester than at the production point(exit) of the individual heat harvester, the rock may cool more at theentrance than the exit of each heat harvester and develop a cone-shapedthermal depression. Operating two adjacent heat harvesters such that theinjection and exit points alternate may allow the two heat harvesters tobe spaced closer together (e.g., since the funnels of the thermaldepression cones may not overlap).

Underground heat harvesting may be accomplished by drilling multipleheat harvesters from a common entrance and/or exit portion (also “mainentrance and/or exit portion” and “single entrance and/or exit portion”herein). Such deviated configurations may allow drilling costs to bedecreased or minimized. For example, a common entrance and exit portionmay comprise a common well portion, one or more common connectingportions and/or a common heat harvesting portion. The common entranceand/or exit portion may be near a target heat exchange region, anddeviating portions (also “deviated portions” herein) of the individualheat harvesters may be drilled from the common entrance and/or exitportion (e.g., to decrease or minimize drilling length).

Two independently drilled portions of the heat harvester (e.g.,comprising two independently drilled well portions) can be connectedwith a coupling to create a complete loop of the underground heatharvester. For example, to allow rapid creation of the underground heatharvester, the entrance (e.g., an entrance well portion or a mainentrance well portion) and the exit (e.g., an exit well portion or amain exit well portion) may be drilled simultaneously. Each well portioncan be part of a separate portion of the heat harvester (e.g., of anentrance portion or an exit portion). Individual portions of the heatharvester (e.g., the entrance portion and the exit portion) can eachcomprise a casing. Where the two portions of the heat harvesterconverge, a coupling can be used to complete the joint between the twocasings. The coupling may be positioned anywhere along the undergroundheat harvester, such as, for example, at the target depth or above thetarget depth.

A heat harvester may be drilled into a triangular shape (also “triangleshape” herein). In this configuration, the entrance and exit can beco-located and heat harvester length at the target temperature can beformed by drilling diagonally and then bending the borehole to create alinear stretch or segment of the heat harvester. In the triangle shapeconfiguration, a kickoff depth (e.g., depth where the heat harvestertransitions from a vertical to an angled or horizontal orientation) maybe near the surface, or deeper underground. In some embodiments, atleast a portion of the triangle shaped heat harvester may differ (e.g.,the entrance well portion and the exit well portion can be configureddifferently while the heat harvester remains the same below the kickoffdepth).

FIG. 2 is an elevation view of an example triangle configuration withina target formation (e.g., rock comprising a region suitable forgeothermal heat harvesting). A primary fluid is injected at location 201and enters a triangle underground heat harvester 200. The enteringprimary fluid may be warmer than surrounding rock 210. A thermallyinsulating cement or grout 202 may be used to decrease or prevent heatloss until, at a heating depth 204, the geothermal gradient results in arock temperature that is higher than the entering primary fluidtemperature. The borehole is kicked off (e.g., directed at an angle ofat least about 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°,60°, 65°, 70°, 75°, 80°, 85° or 90° with respect to the verticaldirection) at a given depth (also “kickoff depth” herein) or position203 to a buildup angle leading up to a target temperature (e.g.,temperature of the rock at a given depth, such as, for example at atarget depth, or temperature of the rock at a range of depths). Theangle (also “buildup angle” herein) may or may not be constant betweenthe kickoff depth and the target temperature depth(s). The buildup anglecan be as sharp as allowed by state of the art drilling technology. Assuch, the angles shown in FIG. 2 may not be to scale. The kickoff depth203 may be at least about 5%, 10%, 25%, 50% or 75% less than the heatingdepth 204. The kickoff depth may be about equal to the heating depth.The kickoff depth may be at least about 5%, 10%, 25%, 50% or 75% greaterthan the heating depth. A first sharp turn (e.g., with an inside angleof at least about 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°,55°, 60°, 65°, 70°, 75°, 80°, 85° or) 90° may be introduced at location205 where the heat harvester 200 reaches a primary heat transfer regionat the target temperature and depth.

The heat harvester 200 extends through the target formation, and may usea thermally conductive cement or grout 207 along at least a portion(e.g., along greater than or equal to about 20%, 40%, 60%, 80%, 90% or100%) of the primary heat transfer region. The heat harvester then makesa second sharp turn at location 213 (e.g., with an inside angle of atleast about 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°,60°, 65°, 70°, 75°, 80°, 85° or 90°) and builds up angle toward an exitof the heat harvester. A thermally insulating cement or grout 208 (e.g.,same or different type than the insulating cement or grout 202) may beused (e.g., reintroduced) to decrease or prevent heat loss from theheated primary fluid into the cooler rock 210. The heated primary fluidexits the underground heat harvester at location 209 and can be used forprocessing (e.g., as discussed with reference to FIG. 1).

The heat harvester 200 can be drilled as two separate portions thatintersect underground and are connected with a coupling 206 (e.g.,respective casings of the two separate portions can be connected via thecoupling). The coupling may be positioned anywhere along the undergroundheat harvester (e.g., at the target depth or above the target depth). Inan example, the coupling may be located anywhere along a horizontalportion 212. In another example (not shown in FIG. 2), only one portionof the heat harvester is horizontally drilled and makes approximately a110° turn from its horizontal length to meet with the second portion ofthe heat harvester which is drilled vertically.

With continued reference to FIG. 2, an entrance well portion of the heatharvester 200 may comprise the portion between the entrance or inletlocation 201 and the kickoff depth 203. At least a portion (e.g.,greater than or equal to about 20%, 40%, 60%, 80%, 90% or 100%) of theentrance well portion may comprise the insulating cement or grout 202. Afirst connecting portion of the heat harvester 200 may comprise theportion between the kickoff depth 203 and the heating depth 204. Atleast a portion (e.g., greater than or equal to about 20%, 40%, 60%,80%, 90% or 100%) of the first connecting portion may comprise theinsulating cement or grout 202. A heat harvesting portion of the heatharvester 200 may comprise the portion between the heating depth 204 andan insulation depth 211. At least a portion (e.g., greater than or equalto about 20%, 40%, 60%, 80%, 90% or 100%) of the heat harvesting portionmay comprise the thermally conductive cement or grout 207. A secondconnecting portion of the heat harvester 200 may comprise the portionbetween the insulation depth 211 and the kickoff depth 203. At least aportion (e.g., greater than or equal to about 20%, 40%, 60%, 80%, 90% or100%) of the second connecting portion may comprise the insulatingcement or grout 208. An exit well portion of the heat harvester 200 maycomprise the portion between the kickoff depth 203 and the exit oroutlet location 209. At least a portion (e.g., greater than or equal toabout 20%, 40%, 60%, 80%, 90% or 100%) of the exit well portion maycomprise the insulating cement or grout 208. Together, the entrance wellportion and the exit well portion may form a well portion. The wellportion may comprise multi-stage casing (e.g., cemented or grouted inconventional way). At least a portion of the heat harvesting portion cancollect or harvest the heat from the surrounding rock. The remaining(non-heat harvesting) portions of the heat harvester 200 may neitherlose heat nor collect or harvest any additional heat. An entranceportion of the heat harvester 200 may comprise, for example, theentrance well portion and the first connecting portion, or the entrancewell portion, the first connecting portion and at least a portion of theheat harvesting portion. An exit portion of the heat harvester 200 maycomprise, for example, the second connecting portion and the exit wellportion, or at least a portion of the heat harvesting portion, thesecond connecting portion and the exit well portion. Entrance and exitportions of the heat harvester may be symmetric such that the buildupangles and first and second sharp turns 205 and 213 are equal and/orsuch that the kickoff depths are equal. Alternatively, the entrance andexit portions may have different angles, turns and/or different kickoffdepths.

The heat harvester 200 may be arranged in a once-through configuration(e.g., not pipe-in-pipe or U-bend). The heat harvester 200 may comprisea closed loop (e.g., no free migration through rock). The heat harvester200 may comprise co-located well heads (e.g., allowing small surfacesite footprint). The heat harvester 200 may be directionally drilled(e.g., comprising significant horizontal length). Heat extracted usingthe heat harvester 200 may be used for industrial use. The heatharvester 200 may comprise two connected casings (e.g., connected with acoupling). The heat harvester 200 may comprise insulating and/orconductive cement/grout (e.g., versus standard cement/grout).

FIG. 3 is an isometric view of a triangle configuration with multipleindividual heat harvesters 307 in a field configuration 300. Individualheat harvesters 307 may be as described elsewhere herein (e.g., asdescribed in relation to heat harvester 200 in FIG. 2). Multiple heatharvesters can be installed to extract heat from a large region of rock(e.g., from a large primary heat transfer region). In this example,multiple heat harvesters 307 are installed adjacent to each other with acommon drill header 301 to centralize drilling and surface plantoperations. The common drill header 301 may comprise entrance and exitwell portions of the individual heat harvester 307 (e.g., one pair pereach heat harvester 307). Individual heat harvesters 307 may havesubstantially the same or different configurations. For example,individual heat harvesters may have substantially the same or differentkickoff depths 306. Once the collection of heat harvesters 307 reach aprimary heat transfer region 302 (e.g., at a target depth 305), they canbe linearly drilled with a spacing 304 (e.g, between about 10 meters (m)and about 1 kilometer (km), or between 50 m and 1 km, or at least 10 m,20 m, 30 m, 40 m, 50 m, 100 m, 150 m, 200 m, 250 m, 300 m, 350 m, 400 m,450 m, 500 m, 550 m, 600 m, 650 m, 700 m, 750 m, 800 m, 850 m, 900 m,950 m or 1 km apart). The spacing 304 may be kept constant or varied.Within each heat harvester 307, working fluid may flow (e.g., be pumped)in a counter-flow configuration 303 (e.g., such that the working fluidsin adjacent portions of the heat harvesters flow in oppositedirections). The counter-flow configuration may reduce an axial thermalgradient.

The configuration 300 may comprise multiple heat harvesters operatingtogether (e.g., important or necessary for economics). The configuration300 may comprise a target heat exchange zone with significant length(e.g., of the heat harvesters 307) at depth. The configuration 300 maybe arranged in a counter-flow configuration (e.g., to offset axialcooling). The multi-well header configuration 301 may be arranged in acentral shaft. Illustrative examples of central shaft arrangements areset forth in U.S. Pat. No. 8,020,382, hereby incorporated by referenceherein in its entirety.

A single triangle shape heat harvester may be extended into a radiatorshaped multi-heat harvester. In this configuration, a single entranceportion can be drilled (e.g., at least in part at an angle) untilreaching a given temperature and/or a given depth (e.g., until reachinga temperature within less than or equal to about 0%, 1%, 5%, 10%, 15%,20%, 25%, 30%, 40% or 50% of the target temperature, and/or untilreaching a depth within less than or equal to about 0%, 1%, 5%, 10%,15%, 20%, 25%, 30%, 50% or 75% of the target depth). Several deviatedportions of individual heat harvesters (e.g., comprising heat harvestingportions of the individual heat harvesters) can then be drilleddirectionally from the single entrance portion. Such deviated portionscan extend through the length of rock near the target temperature andthen re-converging to a single exit portion (e.g., an angled portion ofthe single exit portion).

FIG. 4 is an isometric view of a triangle configuration 400 withportions of individual heat harvesters 407 deviating from commonentrance and exit portions. The configuration 400 can be referred to asa radiator configuration. The radiator configuration may be used toreduce the amount of drilling required to create a heat transfer area ina primary heat transfer region 403. The configuration 400 may compriseseveral deviating portions of individual heat harvesters 407 (e.g., allconnected as a closed loop) from the main entrance and exit portion(e.g., to reduce drilling length). At least one of the heat harvesters407 may be configured as the heat harvester 200, with the remaining heatharvesters 407 deviating away from the plane of the heat harvester 200.

In this example, a single pair of entrance and exit portions 401 may bedrilled, with multiple portions of individual heat harvesters 407deviating at a given depth (also “deviation depth” herein) or position402 from the main entrance and exit portions and extracting heat fromthe primary heat transfer region 403. The deviating portions ofindividual heat harvesters 407 can be linearly drilled with a spacing406 (e.g, between about 10 m and about 1 km, or between 50 m and 1 km,or at least 10 m, 20 m, 30 m, 40 m, 50 m, 100 m, 150 m, 200 m, 250 m,300 m, 350 m, 400 m, 450 m, 500 m, 550 m, 600 m, 650 m, 700 m, 750 m,800 m, 850 m, 900 m, 950 m or 1 km apart) in the primary heat transferregion 403 (e.g., at a target depth 404). The deviating portions ofindividual heat harvesters 407 may be spaced to prevent cooling overlap.The spacing 406 may be kept constant or varied. Since working fluidflows in this example may not be configured in a counter-flowconfiguration, working fluid flow through the heat harvesters 407 may beperiodically reversed to offset an axial thermal gradient.

The pair of entrance and exit portions 401 may have a kickoff depth 405.The kickoff depth 405 for the entrance and exit wells may or may not bethe same. The deviation depth 402 for the entrance and exit portions mayor may not be the same. The deviation depth may be greater than or equalto about 1, 2, 3, 4, 5, 6, 8 or 10 times the kickoff depth (e.g.,located along an angled portion of the entrance and/or exit portion).The deviation depth may be within less than or equal to about 0%, 1%,5%, 10%, 15%, 20%, 25%, 30%, 50% or 75% of the target depth.

The portions of the individual heat harvesters may deviate from the mainentrance and/or exit portions at an angle of, for example, at leastabout 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°,65°, 70°, 75°, 80°, 85° or 90° with respect to a direction of the mainentrance and/or exit portions at the deviation depth. The deviationangle from the entrance portion and the deviation angle from the exitportion may or may not be the same. The deviation depth and/or deviationangle may be the same or different for different individual heatharvesters 407.

The pair of common entrance and exit portions 401 may comprise a commonwell portion 408. The common entrance and exit portions may each furthercomprise a common connecting portion and, in some cases, a common heatharvesting portion. The deviation depth 402 for an individual heatharvester 407 may be located in the common connecting portion or in thecommon heat harvesting portion. The deviating portion of the individualheat harvester 407 may comprise a heat harvesting portion of theindividual heat harvester and, in some cases, one or more connectingportions of the individual heat harvester. Between the deviation depth402 and the target depth 404 (on the entrance portion of the heatharvester 407), the deviating portion of the individual heat harvester407 may comprise, for example, a heat harvesting portion and/or aconnecting portion. At the target depth 404, the deviating portion ofthe individual heat harvester 407 may comprise a heat harvestingportion. Between the target depth 404 and the deviation depth 402 (onthe exit portion of the heat harvester 407), the deviating portion ofthe individual heat harvester 407 may comprise a heat harvesting portionand/or a connecting portion. The deviating portions of the individualheat harvesters 407 may be configured in accordance with respectiveheating depth(s) and/or insulation depth(s) (e.g., as describedelsewhere herein, for example, in relation to FIG. 2). In some examples,the heating depth may be less than or equal to the deviation depth. Insome examples, the heating depth may be greater than the deviationdepth. In some examples, the insulation depth may be less than or equalto the deviation depth. In some examples, the insulation depth may begreater than the deviation depth.

A set of heat harvesters may be drilled in a flower petal shape, withindividual heat harvesters forming individual flower petals. Anindividual heat harvester may comprise entrance and exit portionsdrilled vertically or at an angle (e.g., at a slight angle, such as, forexample, at an angle of at least about 0°, 5°, 10°, 15°, 20°, 25°, 30°,35°, 40° or 45° with respect to the vertical direction) until reaching atarget depth. The heat harvester may comprise a heat harvesting portionat the target depth. The entrance and exit portions can be located atthe center of the flower petal shape. The entrance and exit portions cancomprise, for example, a well portion, one or more connecting portionsand, in some cases, at least a portion of the heat harvesting portion. Aheating depth and an insulation depth may be located along the lengthsof the entrance and exit portions. The heat harvester maybe configuredin accordance with the heating depth and/or insulation depth (e.g., asdescribed elsewhere herein, for example, in relation to FIG. 2). Suchconfiguration may include, for example, use of conductive and/orinsulating cement/grout. For example, the individual heat harvester maycomprise a heat harvesting portion that extends down from the heatingdepth in the entrance portion, through a primary heat transfer region(e.g., at the target depth) and up to the insulation depth in the exitportion.

At or near the target depth, a first borehole (of the entrance portion)can be kicked off horizontally, drilled radially away from the center,and then drilled at an arc. A second borehole (of the exit portion) canbe similarly drilled to finish the arc and complete the loop. A casingmay be placed in each of the first and second boreholes. The first andsecond boreholes may be part of separate portions of the individual heatharvester. Such portions of the heat harvester may be joined by acoupling (e.g., the respective casings of the portions can be connectedvia the coupling). The coupling may be positioned anywhere along theunderground heat harvester (e.g., at the target depth or above thetarget depth). For example, the coupling may be located anywhere alongthe radial and/or arc segments (e.g., along the heat harvesting portionat the target depth). In some embodiments, at least a portion of theindividual flower petal shaped heat harvester may differ (e.g., theentrance and exit portions may be configured differently while the heatharvesting portion at the target depth remains substantially the same).

FIG. 5 is an isometric view of a flower petal configuration 500 withmultiple individual heat harvesters 506 in a field configuration.Individual heat harvesters 506 may be as described elsewhere herein(e.g., as described in relation to individual heat harvesters formingindividual flower petals). Individual heat harvesters 506 may havesubstantially the same or different configurations. The configuration500 may allow the amount of drilling required to create a heat transferarea in a primary heat transfer region 502 to be reduced (e.g., drillinglength to reach target temperature can be minimized). The configuration500 can comprise one or more (e.g., at least 1, 2, 3, 4, 5, 6, 7, 8, 9or 10) individual heat harvesters 506, each comprising a flower petalshaped heat harvesting portion in the primary heat transfer region 502(e.g., at a target depth 507). For example, the configuration 500 cancomprise 4 individual heat harvesters 506, each comprising a flowerpetal shaped heat harvesting portion in the primary heat transfer regionthat spans a quarter circle. The heat harvesters 506 may be spaced todecrease or prevent cooling overlap.

In this example, multiple entrance and exit portions 501 (e.g., one pairper each heat harvester 506) are drilled until reaching the targettemperature and depth zone 502 (e.g., a target depth 507). The entranceand exit portions 501 can be located at the center of the configuration500. Each pair of entrance and exit portions can comprise a wellportion. The multiple well portions of the configuration 500 can beinstalled adjacent to each other using a multi-well headerconfiguration. The multi-well header configuration may be arranged in acentral shaft. Illustrative examples of central shaft arrangements areset forth in U.S. Pat. No. 8,020,382, hereby incorporated by referenceherein in its entirety.

Each heat harvester can then be drilled radially along segment 503(e.g., outward from the entrance and exit portions 501), arc (e.g., fora given distance) along segment 504 and then return radially to thecenter of the system along segment 505. Angles of curvature in theprimary heat transfer region 502 (e.g., at least about 0°, 5°, 10°, 15°,20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85° or90°) can be as sharp as allowed by state of the art drilling technology.As such, the angles shown in FIG. 5 may not be to scale. Radii ofcurvature in the primary heat transfer region 502 may include buildangles of between about 1 and 10 degrees per 100 feet (e.g., at leastabout 1°/100 feet, 2°/100 feet, 3°/100 feet, 4°/100 feet, 5°/100 feet,6°/100 feet, 7°/100 feet, 8°/100 feet, 9°/100 feet or 10°/100 feet). Theheat harvesters 506 may or may not have the same angle and/or curvature.At least a portion (e.g., all) of the heat harvesters may havesubstantially similar angle and/or curvature, or at least a portion ofthe heat harvesters may have one or more different angles and/orcurvatures.

The collection of heat harvesters 506 may or may not be arranged in acommon horizontal plane in the primary heat transfer region 502. Thecollection of heat harvesters 506 may deviate from a common horizontalplane to accommodate given (e.g., particular) geological or drillingdetails. Working fluid may flow in adjacent heat harvesters in acounter-flow configuration (e.g., such that the working fluids inadjacent portions of the heat harvesters flow in opposite directions).The counter-flow configuration may reduce or minimize an axial thermalgradient.

Multiple heat harvesters may be installed to form a flower petal shapein a deviated configuration, where multiple deviating portions of theindividual heat harvesters extend from a main entrance portion, arc at adistance from the entrance portion, and then radially return tore-converge at a main exit portion.

FIG. 6 is an isometric view of a flower petal configuration 600 withportions of individual heat harvesters 604 deviating from commonentrance and exit portions 601 and 602. The portions of the individualheat harvesters 604 may deviate radially toward a horizontal plane. Theconfiguration 600 may comprise multiple deviated portions of individualheat harvesters 604 (e.g., all connected as a closed loop) from the mainentrance and exit portion (e.g., to reduce drilling length). By creatingdeviated portions of individual flower petal shaped heat harvesters 604from a main entrance portion 601 and a main exit portion 602, the amountof drilling required to create a heat transfer area in a primary heattransfer region 603 (e.g., at a target depth 605) may be furtherreduced.

The portions of the heat harvesters 604 may deviate from the mainentrance portion 601 and a main exit portion 602 at a deviation depth.The deviation depth for the entrance and exit portions may or may not bethe same. The deviation depth may be within less than or equal to about0%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 50% or 75% of the target depth. Thedeviation depth may be the same or different for different individualheat harvesters 604. The portions of the individual heat harvesters maydeviate from the main entrance and/or exit portions at an angle of, forexample, at least about 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°,50°, 55°, 60°, 65°, 70°, 75°, 80°, 85° or 90° with respect to adirection of the main entrance and/or exit portions at the deviationdepth. The deviation angle may be as steep as allowed by directionaldrilling technology. The deviation angle may be configured to decreaseor minimize the amount of drilling length necessary to get to ahorizontal orientation (e.g., the bend may be tight). The deviationangle from the entrance portion and the deviation angle from the exitportion may or may not be the same. The heat harvesters may deviate inan arc (e.g., not at a constant angle). The deviation may have acurvature (e.g., a radius of curvature specified in terms of a buildangle). The deviation may have a build angle of between about 1 and 10degrees per 100 feet (e.g., at least about 1°/100 feet, 2°/100 feet,3°/100 feet, 4°/100 feet, 5°/100 feet, 6°/100 feet, 7°/100 feet, 8°/100feet, 9°/100 feet or 10°/100 feet). The deviation curvature from theentrance portion and the deviation curvature from the exit portion mayor may not be the same.

The common entrance and exit portions 601 and 602 may comprise a commonwell portion. The common entrance and exit portions 601 and 602 may eachfurther comprise a common connecting portion and, in some cases, acommon heat harvesting portion. The deviation depth for an individualheat harvester 604 may be located in the common connecting portion or inthe common heat harvesting portion. The deviating portion of theindividual heat harvester 604 may comprise a heat harvesting portion ofthe individual heat harvester and, in some cases, one or more connectingportions of the individual heat harvester. Between the deviation depthand the target depth 605 (on the entrance portion of the heat harvester604), the deviating portion of the individual heat harvester 407 maycomprise, for example, a heat harvesting portion and/or a connectingportion. At the target depth 605, the deviating portion of theindividual heat harvester 604 may comprise a heat harvesting portion.Between the target depth 605 and the deviation depth (on the exitportion of the heat harvester 604), the deviating portion of theindividual heat harvester 604 may comprise a heat harvesting portionand/or a connecting portion. The deviating portions of the individualheat harvesters 604 may be configured in accordance with respectiveheating depth(s) and/or insulation depth(s) (e.g., as describedelsewhere herein, for example, in relation to FIG. 2). In some examples,the heating depth may be less than or equal to the deviation depth. Insome examples, the heating depth may be greater than the deviationdepth. In some examples, the insulation depth may be less than or equalto the deviation depth. In some examples, the insulation depth may begreater than the deviation depth.

Angles of curvature in the primary heat transfer region 603 (e.g., atleast about 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°,60°, 65°, 70°, 75°, 80°, 85° or 90°) can be as sharp as allowed by stateof the art drilling technology. As such, the angles shown in FIG. 6 maynot be to scale. Radii of curvature in the primary heat transfer region603 may include build angles of between about 1 and 10 degrees per 100feet (e.g., at least about 1°/100 feet, 2°/100 feet, 3°/100 feet, 4°/100feet, 5°/100 feet, 6°/100 feet, 7°/100 feet, 8°/100 feet, 9°/100 feet or10°/100 feet).

The heat harvesters 604 may or may not have the same angle and/orcurvature. At least a portion (e.g., all) of the heat harvesters mayhave substantially similar angle and/or curvature, or at least a portionof the heat harvesters may have one or more different angles and/orcurvatures.

The collection of heat harvesters 604 may or may not be arranged in acommon horizontal plane in the primary heat transfer region 603. Thecollection of heat harvesters 604 may deviate from a common horizontalplane to accommodate given (e.g., particular) geological or drillingdetails. The heat harvesters can collect heat from the primary heatexchange region 603, where adjacent heat harvesters can be operated in acounter-flow configuration (e.g., such that the working fluid inadjacent portions of the heat harvesters flows in opposite directions).The counter-flow configuration may reduce an axial thermal gradient.

Thermal conductivity may be a critical component of the overallperformance of the heat harvester. Boreholes may be directionallydrilled through rock with known high conductivity to increase ormaximize thermal production. For example, high conductivity veins ofrock can be targeted with directional drilling to increase or maximizethe heat extraction rate. Rock may be targeted selectively.

FIG. 7 is an elevation view of a closed loop heat harvester 700directionally drilled through a vein of rock with high thermalconductivity. A high thermal conductivity vein of rock 701 can betargeted with one or more triangular shaped heat harvesters 700 (e.g.,triangular shaped heat harvester 200 in FIG. 2). A similar strategy canbe used to target a high thermal conductivity disc of rock using one ormore flower petal shaped heat harvesters (e.g., collection of flowerpetal shaped heat harvesters 506 in FIG. 5 or 604 in FIG. 6). Thermallyconductive rock may be targeted (e.g., to improve or optimizeperformance within a given rock formation). The heat harvester(s) 700may drilled through virgin rock (e.g., not through an oil and gas well).

The rock immediately around the heat harvester (e.g., around the heatharvesting portion of the heat harvester) may be at its nativetemperature (e.g., at least about 100° C.) at the beginning of life(BOL), but may rapidly cool after the introduction of cold working fluid(e.g., cold water). A power spike may occur at the BOL (e.g., duringextraction of heat from non-porous rock). The heat extracted by theharvesting portion after the rapid cooling phase may be driven byconduction through the rock. The amount of heat extracted by the heatharvesting portion after the rapid cooling phase may exponentiallydecrease and level off over time (e.g., after a few years). The rock maydevelop a thermal depression around the heat harvesting portion (e.g.,as a result of the rapid cooling). By adjusting the fluid flow rateentering the underground heat harvester, heat extraction rate may beleveled over life (e.g., such that the heat extraction rate is suitablefor operation of a power plant configured for baseload power). Such flowcontrol may be achieved by a variable speed pump, by throttling pumpflow with a throttling valve and/or by other suitable approaches. In anexample, flow control over life may comprise providing a low flow rateat the BOL of the heat harvester and gradually increasing the flow rateover the lifetime of the heat harvester. Flow rates may range frombetween about 5 kg/s and 100 kg/s, or be at least 1 kg/s, 5 kg/s, 10kg/s, 15 kg/s, 20 kg/s, 25 kg/s, 30 kg/s, 35 kg/s, 40 kg/s, 45 kg/s, 50kg/s, 55 kg/s, 60 kg/s, 65 kg/s, 70 kg/s, 75 kg/s, 80 kg/s, 85 kg/s, 90kg/s, 95 kg/s or 100 kg/s.

Cement or grout can fill the annular space between the casing and therock (e.g., the space between the outer diameter of the casing and theborehole). The cement or grout may be modified to have insulatingproperties or conductive properties. Thermally conductive cement orgrout (also “conductive cement/grout” herein) can increase or improvethe rate of energy extraction from the target rock (e.g., increase orimprove flow of heat into the heat harvester in a primary heat transferregion). Thermally insulating cement or grout (also “insulatingcement/grout” herein) can decrease or minimize heat loss from the heatharvester (e.g., decrease or prevent loss of heat whenever the fluid ishotter than the surrounding rock). Insulating cement/grout may be usedto decrease or prevent heat loss from entering fluid (e.g., workingfluid that has not been heated and is flowing in a direction from theentrance at the surface toward the target depth) when it is at a greatertemperature than the surface rock (e.g., rock at or near the surface orat a depth substantially less than the target depth). Insulatingcement/grout may be used to prevent heat loss from exiting heated fluidwhich is rising to the surface to exit the underground heat harvester(e.g., working fluid that has been heated and is flowing in a directionfrom the target depth toward the exit at the surface).

Systems of the disclosure may be applied to perform various geothermalheat harvesting methods. For example, systems of the disclosure may beused to transfer heat extracted from the an underground heat harvester(e.g., using the system of FIG. 1), to drill underground heat harvestersin various configurations (e.g., see FIGS. 2-7), to extract heat from alarge region of rock by drilling multiple heat harvesters (e.g., usingthe system of FIG. 3), to reduce (e.g., to varying degrees) the amountof drilling required to create a heat transfer area in a primary heattransfer region (e.g., using the systems of FIG. 4, 5 or 6), to target ahigh thermal conductivity vein of rock with one or more heat harvesters(e.g., using the system of FIG. 7), and so on.

Different aspects of the invention can be appreciated individually,collectively, or in combination with each other. Further, variousaspects of the disclosure may be advantageously adapted to differentheat harvester configurations. For example, the heat harvester maycomprise a coaxial pipe-in-pipe (e.g., closed loop pipe-in-pipe), whichmay decrease or minimize drilling costs by pumping fluid through anouter annulus of the well with the heated fluid returning through aninsulated center pipe. Such a heat harvester may be installed in atriangle shape or a flower petal shape. Various aspects of thedisclosure may be advantageously applied in open loop configurations,closed loop U-bend and/or otherwise shaped (e.g., otherwise shapedonce-through) configurations, configurations with underground equipment,closed loop configurations with augmented rock, oil and gas wells,and/or other configurations.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. It should be understoodthat various alternatives to the embodiments of the invention describedherein may be employed in practicing the invention. It is intended thatthe following claims define the scope of the invention and that methodsand structures within the scope of these claims and their equivalents becovered thereby.

What is claimed is:
 1. A geothermal heat harvesting system, comprising:a geothermal heat harvester comprising an entrance and an exit at asurface, wherein the entrance and the exit are in fluid communicationvia a path, the path comprising: a first segment extending between thesurface and a target depth, the first segment comprising the entrance; asecond segment in fluid communication with the first segment andextending radially outward at the target depth, the second segment beingsubstantially perpendicular to the first segment; a third segment influid communication with the second segment and extending in an arc atthe target depth; a fourth segment in fluid communication with the thirdsegment and extending radially inward at the target depth; and a fifthsegment extending between the target depth and the surface, the fifthsegment comprising the exit and being substantially perpendicular to thefourth segment, wherein the second segment, the third segment, and thefourth segment (i) are substantially planar at the target depth and (ii)together comprise a primary heat transfer region, wherein the firstsegment and the fifth segment are each substantially vertical, andwherein the second segment and the fourth segment are each substantiallyhorizontal.
 2. The geothermal heat harvesting system of claim 1, whereinthe path comprises two independently drilled portions connected with acoupling.
 3. The geothermal heat harvesting system of claim 2, whereinthe coupling is at the target depth.
 4. The geothermal heat harvestingsystem of claim 2, further comprising: a sixth segment between the firstsegment and the second segment, the sixth segment deviating from thefirst segment toward the second segment; and a seventh segment betweenthe fourth segment and the fifth segment, the seventh segment deviatingfrom the fifth segment toward the fourth segment.
 5. The geothermal heatharvesting system of claim 4, further comprising an additionalgeothermal heat harvester deviating from the geothermal heat harvester,the additional geothermal heat harvester comprising: an eighth segmentin fluid communication with the first segment; a ninth segment in fluidcommunication with the eighth segment and extending radially outward atthe target depth, the ninth segment being substantially perpendicular tothe first segment and the eighth segment deviating from the firstsegment toward the ninth segment; a tenth segment in fluid communicationwith the ninth segment and extending in an arc at the target depth; aneleventh segment in fluid communication with the tenth segment andextending radially inward at the target depth, the eleventh segmentbeing substantially perpendicular to the fifth segment; and a twelfthsegment in fluid communication with the eleventh segment and the fifthsegment, the twelfth segment deviating from the fifth segment toward theeleventh segment.
 6. The geothermal heat harvesting system of claim 5,wherein the ninth segment and the eleventh are each substantiallyhorizontal.
 7. The geothermal heat harvesting system of claim 6, whereinhorizontal portions of the geothermal heat harvester and the additionalgeothermal heat harvester are rotated by about 45°, 90°, or 135° fromeach other.
 8. The geothermal heat harvesting system of claim 1, whereinthe path forms a closed loop.
 9. The geothermal heat harvesting systemof claim 1, wherein the entrance and the exit are co-located.
 10. Thegeothermal heat harvesting system of claim 9, wherein the path comprisestwo portions, wherein a first portion of the two portions comprises awell head that forms the entrance, and wherein a second portion of thetwo portions comprises a well head that forms the exit.
 11. Thegeothermal heat harvesting system of claim 1, further comprisinginsulating cement or grout disposed along at least a portion of thepath, conductive cement or grout disposed along at least a portion ofthe path, or a combination thereof.
 12. The geothermal heat harvestingsystem of claim 1, further comprising a primary fluid that flows throughat least a portion of the path, wherein the primary fluid is configuredto enter at the entrance and exit at the exit, and wherein a flow rateof the primary fluid is configured for control over a life of thegeothermal heat harvester such that a heat extraction rate from thegeothermal heat harvester is leveled through the life.
 13. Thegeothermal heat harvesting system of 1, further comprising an additionalgeothermal heat harvester operating together with the geothermal heatharvester in a field configuration, the additional geothermal heatharvester comprising a separate path with substantially the sameconfiguration as the path of the geothermal heat harvester.
 14. Thegeothermal heat harvesting system of claim 13, wherein the additionalgeothermal heat harvester is adjacent to the geothermal heat harvester,wherein the geothermal heat harvester is configured to circulate a firstprimary fluid and the additional geothermal heat harvester is configuredto circulate a second primary fluid in a counter-flow configuration withrespect to the first primary fluid.
 15. The geothermal heat harvestingsystem of claim 13, wherein the additional geothermal heat harvester andthe geothermal heat harvester are spaced to prevent cooling overlap.